I read the OP and I still stick to my guns (no pun intended). No weapon, attack, or ability should ever, ever be capable of a one-hit kill in Brink.
Except the devs have already said a "maybe" (which has seen a shift to everyone saying it's confirmed over the past couple of weeks) to a Long Rifle one-shotting an unbuffed Light on a headshot. IF you're using the bolt-action Long Rifle without a silencer (presumably reduced damage if you are) AND your target is Light, AND they passed on the "Battle Hardened" ability, AND they're yet to meet a friendly Medic, AND their team have no Health CPs, AND you're good enough to get a headshot...
There's a lot more "if" there than "if you hit the body with a molotov." And for mines, the best you'll get is "IF their Operative hasn't marked it (likely they won't have), AND they step on it, AND they either a) are moving to fast to stop, or B) decide not to wait for someone to disarm" - mines only need 3 conditions to land their damage, and yes, they should be high damage + knockdown, but instant incapactitation is too much. I'd say MAYBE on an unbuffed Light, and MAYBE again for grenades/molotovs on unbuffed Lights, but preferably not for mines or grenades, because they get knockdown effects - if you're close, or a teammate is, the melee follow-up will be your incap.
Overall, a general "No OHK" rule for the game is a good idea because of how the game is set up. I'm not saying they should stick to that even if the result is less fun than expected. If everyone comes back after playing and says "all the weapons feel like toys and it's silly" even when we were supporting the "no OHK" policy at first, then yes, they probably will change that. But for now, I think it's a great idea, and one well worth the effort of exploring. I'm fairly confident the game will be better because of the survivability that is so often lacking in shooters these days.